refactor(tui): status icons, clean STATUS column, relative time

- STATUS column shows icon + clean state only (▲ UP, ▼ DOWN, ◆ LATE,
  ◆ STALE, ◇ PAUSED, ◼ MAINT, ○ PENDING). Error classification
  (DNS/TLS/TMO) removed from STATUS — stays in NAME inline hint.
- Detail panel Last Check shows relative time ("12s ago") instead of
  absolute timestamp.
- Extract shared fmtTimeAgo() to format.go, consolidate duplicate
  formatters in tab_alerts.go and tab_nodes.go.

internal/tui/tab_sites.go

@@ -128,7 +128,7 @@ func (m Model) viewSitesTab() string {
 						strconv.Itoa(i + 1),
 						m.zones.Mark(fmt.Sprintf("site-%d", i), icon+" "+limitStr(site.Name, nameW-4)),
 						"group",
-						fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID), ErrCatUnknown),
+						fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)),
 						subtleStyle.Render("—"),
 						m.groupUptime(site.ID),
 						m.groupSparkline(site.ID, sparkWidth),
@@ -175,7 +175,7 @@ func (m Model) viewSitesTab() string {
 					strconv.Itoa(i + 1),
 					m.zones.Mark(fmt.Sprintf("site-%d", i), name),
 					typeIcon(site.Type, false) + " " + site.Type,
-					fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID), classifyError(site.LastError, site.Type, site.StatusCode)),
+					fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)),
 					fmtLatency(site.Latency),
 					fmtUptime(hist.Statuses),
 					spark,